This invention relates to a method for producing ultrafine powder from a metal or metal alloy, and more particularly to such a method involving atomization of a stream of molten metal or metal alloy by an impinging cone of atomizing gas.
It is known to pass a stream of molten metal through a nozzle and to direct one or more high velocity jets of gas at the emerging stream to break up the stream into small droplets which solidify into particulates of varying sizes. Such gas atomization techniques are valuable for the production of prealloyed (multicomponent) systems as spherical particles which are clean and have low oxygen and nitrogen contents. However, a major disadvantage of prior art methods is the low yield of fine powders that can be obtained. The particle size distribution also tends to be broad, making control of the yield of fine powders even more difficult.
There is at present a growing industrial demand for ultrafine metal powders, i.e. powders having a particle diameter smaller than 10 microns. Presently only about 1% to 3% of the particles of industrially produced powder is within this ultrafine size range, making the cost of such powders verh high. Accordingly, there is a need to develop gas atomization techniques which can increase the yield of such ultrafine powder, and to narrow the particle size distribution.
The diameter of the particles and the size distribution are influenced by the surface tension of the melt from which the powder is produced. For melts of high surface tension, for example copper and copper alloys, production of fine powder is more difficult and consumes more gas and more energy.
Methods for the production of fine powder find particular usefulness in the field of rapid solidification materials. It is known that the rate of solidification of a molten particle of relatively small size in a convective environment such as a flowing gas is roughly proportional to the inverse of the diameter of the particle squared. Accordingly, if the average size of the diameter of the particles of the composition is reduced then the rate of cooling is increased dramatically. This property becomes particularly important in the production of amorphous metal and metal alloys. By producing metal powders having a narrow size distribution and a high percentage of ultrafine powders, novel amorphous and related properties may be achieved. Also, novel properties may be achieved in the production of superalloys.
Further, the achievement of smaller particle size and narrow size range can have advantages in the consolidation of materials by conventional powder metallurgy, resulting in a higher packing density and a higher sintering rate.
Recently, much experimentation has been performed to improve the atomization process. For example, various gas nozzles have been developed which use an ultrasonic, pulsed gas flow to atomize the melt. Other researchers have stated that high pressure gas flow directed in such a way as to produce aspiration or low pressure conditions at the melt outlet increases the production of fine powders, the percent of fines increasing with increasing aspiration. However, no method has as yet been successfully adapted to consistently and predictably produce a high percentage of ultrafine metal or metal alloy powder on a commercial scale.